This verse is referring to the incarnation of Jesus Christ, when He took on human flesh and lived among us as both fully God and fully man. The phrase "the mystery of godliness" refers to the nature of Jesus Christ and the truth about who He is and what He came to accomplish. This mystery, which was previously hidden from human understanding, has now been revealed through His life, death, and resurrection.
The rest of the verse lists various aspects of the life and ministry of Jesus that serve as evidence of His divine nature and the truth of the gospel. He appeared in a body, which refers to His physical incarnation; was vindicated by the Spirit, which refers to the supernatural power that was demonstrated in His life and ministry; was seen by angels, which is a reference to the angels who witnessed His birth and resurrection; was preached among the nations, which refers to the spread of the gospel message throughout the world; was believed on in the world, which refers to the growth of the early church; and was taken up in glory, which refers to His ascension into heaven.
In conclusion, it is a declaration of the truth and significance of the incarnation of Jesus Christ and a call to acknowledge and believe in this mystery of godliness.
1 Timothy 3:16. Without controversy, the mystery of godliness is great: God was revealed in the flesh, justified in the spirit, seen by angels, preached among the nations, believed on in the world, and received up in glory.